Changes

tarexport: Plumb ctx, add OTEL spans, handle cancellation

Pass `context.Context` through `tarexport.Load` and `tarexport.Save`.
Create OTEL spans for the most time consuming operations.

Also, handle context cancellations to actually end saving/loading when
the operation is cancelled - before this PR the daemon would still be
performing the operation even though the user already cancelled it.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>